build: use -ftrapv for development

We want to find these bugs if they exist.
man: beef up ExecStart description

We have lots of questions from people who assume that shell syntax works
here, so let's be very explicit what is allowed and what is not. A few
examples should also help.

http://bugs.debian.org/732156
man: remove advice to avoid setting the same var more than once

So far the compatibility with .desktop settings hasn't been imporant
at all, and we do not want people to write convoluted unit
files.